A secondary battery has been popularly used as a power source for electronic equipment such as a mobile phone or IT equipment in addition to an application where a primary battery is replaced with a secondary battery. Particularly, a nonaqueous electrolyte secondary battery represented by a lithium ion secondary battery possesses high energy density and hence, the application of the nonaqueous electrolyte secondary battery to electrical equipment such as an electric vehicle has been in progress.
An electrolyte solution in the secondary battery is decomposed and vaporized as the secondary battery is repeatedly charged and discharged, and the exterior of the secondary battery is expanded or deformed because of such decomposition and vaporization of the electrolyte solution. To prevent such expansion or deformation of the exterior, a safety valve which is opened at a predetermined pressure and discharges a gas in the secondary battery is mounted on the exterior of the secondary battery. Further, an assembled battery, in which a plurality of batteries are arranged and connected to each other, includes an exhaust passage which communicates with safety valves of the respective batteries and guides and discharges a gas to the outside (see JP-A-2013-168355 including FIG. 5 and FIG. 15 for example).